This application claims benefit of priority under 35 USC 119 to Japanese Patent Application No. P2000-194570 filed on Jun. 28, 2000, the entire contents of which are incorporated by reference herein.
1. Field of the Invention
The present invention relates to a semiconductor device with an inductance element.
2. Description of Related Art
Conventional inductance elements are of core type or of shell type and employ combinations of coils and magnetic materials. These types of inductance elements became prevalent over monolithic inductance elements formed on ICs because the monolithic inductance elements i) must be bulky if required inductance is large and ii) must employ magnetic materials that are unsuitable for IC processes.
The reason why the inductance elements employ magnetic materials is because, when the inductance elements are energized, the magnetic materials effectively use magnetic flux to increase inductance.
Recent ICs, however, operate at high frequencies and require low inductance. Due to this and to make devices compact, there are requirements of integrally forming inductance elements on ICs or fabricating multi-chip modules including inductance elements.
To form inductance elements on ICs, it is preferable to make the inductance elements with flat coils such as a meander coil of FIG. 1A and a spiral coil of FIG. 1B. Compared with multilayer coils, the flat coils involve ignorable contact resistance or via resistance and are easy to design.
FIG. 2 is a sectional view showing a resin-sealed semiconductor-device having an inductance element. This device has a conductive bed 101, a mount 102 laid on the bed 101, and a semiconductor chip 103 fixed to the mount 102. A principal plane of the chip 103 has the inductance element 104 made of a spiral coil. FIG. 3 is an enlarged view showing the inductance element 104 on the chip 103. Electrodes of the chip 103 are connected to leads 106 through bonding wires 105. The chip 103, leads 106, and wires 105 are consolidated with resin 107 into a package with the leads 106 partly extending outside.
The problem of this semiconductor device will be explained. In FIG. 3, the spiral coil (inductance element) 104 is formed on the semiconductor chip 103. When current is passed, the coil 104 generates magnetic flux perpendicularly to the coil 104 as shown in FIG. 4 that shows equi-vector potential lines representing the magnetic flux. When the coil 104 is operated at a high frequency by passing high-frequency current through the coil 104, the direction of magnetic flux generated by the coil 104 changes at the current frequency. If a conductor 301 is arranged in the vicinity of the coil 104, the magnetic flux from the coil 104 generates eddy current in the conductor 301, to decrease the inductance of the coil 104.
In FIG. 2, the semiconductor chip 103 is mounted on the conductive bed 101, to form a standard semiconductor package. The distance between the coil 104 and the bed 101 is substantially equal to the thickness of the chip 103. Namely, the inductance element, i.e., the coil 104 is in the vicinity of a conductor, i.e., the bed 101 at the distance defined by the thickness of the chip 103.
FIG. 5 is a graph showing a relationship, between inductance provided by an inductance element operated at 10 MHz and the distance of the inductance element from a Cu conductor. The inductance element is a coil of 4000 xcexcm in diameter, 80 xcexcm in wire width, 80 xcexcm in wire interval, 10 in turns, and 19 xcexcm in thickness. The Cu conductor has a thickness of 0.15 mm. As the distance between the coil and the Cu conductor shortens, the inductance decreases. If the distance becomes 0.6 mm or shorter, the inductance decreases at the rate of the first power of the distance.
In FIG. 2, the bed 101 is a conductor and is close to the inductance element 104 at the distance defined by the thickness of the chip 103. This thickness is thin to deteriorate the inductance of the inductance element 104, as is apparent from FIG. 5. If the thickness of the chip 103 is 0.29 mm, a designed inductance value will decrease to 69%.
The present invention is to provide a semiconductor device capable of reducing eddy current produced in a bed and securing required inductance. A semiconductor device according to this invention includes a semiconductor chip, an inductance element of flat structure formed above a first surface of the semiconductor chip, and a magnetic material formed below a second surface of the semiconductor chip opposite to the first surface. The semiconductor device according to the present invention has a magnetic material formed on the semiconductor surface opposite to the inductance element.